JPTL

My front AC isn't blowing as cold as it should.
Here are today's test conditions:

• Engine at summer operating temp
• AC on recirc.
• Front fan on full
• Rear fan on full
• Front output temp measurement taken at front center vent (open)
• Rear output temp measurement taken at middle vent of rear AC.

Here are the factors:

• Recirc. flap opens fully as it should per description by other members
• System recently serviced, freon level/pressure checked
• Heater valve immediately closes fully and remains closed while AC is on (see pic)
• The rear AC blows about 58º
• The front @ vents blows about 78º

So there's a 20º discrepancy between the front output and the rear when they're both working simultaneously. Given that my vacuum system seems to be operating correctly, can I isolate this to a faulty heater valve & go on the premise that these warp over time and even when the vacuum is operating as it should at the valve (arm fully actuated), the valve is letting hot water pass into the cabin?
Is there something else that can cause the discrepancy between front & rear AC output temps?
Also, does the rear AC rely on the recirculated cabin air for it's air source...i.e. will it provide cooler output if the cabin air temp is reduced?

dr bob

JP--

On the front unit, look for about 40º temp drop between pass side floor and the center vent with fan on second speed. Less than that means you likely have heat coming in from somewhere, assuming that the refrigeration pressures are OK. If there isn't air in the system, the low-side guage tells you the corresponding freon flash temp at your current suction pressure. The difference between those and the center vent temp is a vectored sum of heat loads and an airflow factor. Also: If the center vent temp doesn't change much between low fan speed and high fan speed, you likely have heat coming in from the heater core/valve.

The rear unit draws cabin air in through the grill immmediately aft of the discharge vents on top. Circulates through the evap from top rear to bottom to out at top front. Lots of opportunity for recirculation obviously, and ho firewall heat or heater core heat to add, so temps there tend to be a little lower. It will have cooler output with lower cabin temp, as will the front system on recirc.

Alan

Rear air recirculates only - it has no fresh air feed.

Rear air also uses the same compressor/condenser refrigerant circuit as the front... if there is any blockage in the front evaporator/evap valve then here will be more flow to the rear which will make the front warmer. What happens to the front temps with the rear flow off...?

Warm front could otherwise be a problem with the hot water valve (vacuum or mechanical) + also possibly the mixing flaps and comb flap (though you'd have to have some hot water leakage also).

It also could be the sensor loop or a sensor problem - but this usually leads to uncontrolled heat.

Alan

JPTL

Bob,
Do you mean that the center vent temp should be about 40º less than a temp. measurement on the pass side floor? If that's what you mean, no way in hell was that floor 118º.
I don't have any AC test equip. so I lose you after "the low-side gauge.."
There is a nearly immeasurable change in temp between low and high fan speeds.
..and to answer your question Alan, running the rear AC vs. not running it, results in minimal to no temp. change @ the front. I'm not familiar with evaporator valves, but both expansion valves were replaced recently. Regarding a sensor problem, I haven't had any erratic temp fluctuations in the cooler months, and I watched that heater valve, AC off, heat on, and it seemed to hold steady. As soon as I dialed down the heat to the lowest setting (AC still off) the arm retracted fully, closing the valve and didn't fluctuate. So I assume good vacuum - good sensor (pic of valve below)
Based on what you guys say, I'm leaning toward bad heater valve.
I was wondering how to test the heater valve, and docmirror suggested putting a marble in the line. I like those bohemian testing methods..particularly the cost of the test equipment. After getting a snug marble fit, any decrease in my vent temp. under the same conditions would point to a bad valve, I'd say.
Now to take a look in the kids' toy box for my diagnostic test equipment.
...and thanks guys for taking the time to weigh in. I respect both of your opinions regardless of the topic.

bigred1001

That was my problem. Replaced the heater valve and the front a/c temp dropped about 25 deg or so.

dr bob

That's exactly what I meant. The 40º is a general design figure, not an exacting science. On some cars with mini evaps the 40º is out of reach, but for the 928 it's a good target for broad-brush diagnostic purposes.

The diagnosis on the refrigeration side includes comparing the performance of the liquid/vapor sides of the heat exchangers with what you measure with the thermometer(s) on the air sides. Ambient temp outside is compared with the temp equivalent of the high side pressure to see how much heat is being carried off at the condenser, or more specifically how much heat isn't being carried away. A perfect condenser would have temps inside the same as temps outside.

On the evap side, you can measure the air temp on both sides (approach from the floor, discharge at the center vent) and compare that with the low-side temp equivalent from the pressure gauge. Using these numbers you can see if you are undercharged (not enough heat going away) at a specific evap pressure. You can see if there's a partial load of air in the system by comparing the measured temp with ambient while car is cold in the morning. Bottom line is that there's a lot to be learned from a set of gauges and a couple thermometers.


This points again to a leaking heater valve. The fan would carry more air and cause a rise in temp if there's just the evaporator working. With both the evap and the heater fighting in there, changes in airflow mak a much smaller difference in vent temps.

Grab a pair of long-nosed vise-grips and pinch the hose between the heater valve and the heater core. No muss no fuss, no other tools.

I also proposed in the other thread that you get a set of readings with engine cold in the morning, before the engine has a chance to warm up the coolant. For those playing along at home, this is the quick-and-dirty non-invasive way to discover that your heater valve is leaking. If you get full cooling with the cold engine, it points immediately to a leaking or not-closing heater valve.

WallyP

While the leaking (thru, not outside) heater valve is almost always the culprit, the usual suspects also include maladjusted air control flaps. There is an adjustment procedure in the Factory Workshop Manual.

JPTL

UPDATE:
I found that a wine cork works well to plug the short section of hose before the heater valve. You can tighten down on it with the hose clamp for testing - and summer driving, I'd guess.
I had to drink a bottle of wine in order to get a second cork for the sidewall side of coolant line
After getting the heater valve out, I tested it by simply holding in the vacuum actuator arm, and pouring non-pressurized hot water into the black side. With the valve fully closed, it drained out of the output side of the valve in a second or two...and this was simply with a gravity flow. I have to assume a bit of pressure would increase the rate of leakage.
I haven't had a chance to test the front AC with the blocked line as of yet, but I have to assume that this valve is the culprit.
I can't imagine this amount of leakage being acceptable, so I'll replace the valve.

WallyP

"The rear AC blows about 58º
The front @ vents blows about 78º "

As I have said elsewhere, that is not acceptable perfromance from the refrigeration part of the system. You should be seeing output temps in the 35-45 degree range.

That means that even after you fix the twenty degree variation in the front system, you still won't be cool.

JPTL

Wally - thanks for following my issue here.
I'm thinking that I'm using an inaccurate/uncalibrated thermometer for my measurements. It's consistent in its measurement, but I had doubts about my readings - particularly the rear AC of 58º.
So I tested an AC system known to be good. Best I could do is 55º from the center vent of our '08 Suburban on max/recirc/1,500. That beast cools just fine.
So I'm focused more on the 20º variance (which is probably accurate) - than the actual temp readings - which are likely inaccurate.